The invention discloses a dismounting and mounting method and device of a segmented crankshaft of a marine low speed diesel engine. The device comprises a lower bearing bush fixed tool, a built-in operation platform, a centering tool, a front-and-back crankshaft connecting hydraulic expansion bolt, a crankshaft chain wheel connecting hydraulic expansion bolt, a short stop plate and a long stop plate. The hydraulic expansion bolt is adopted to replace a traditional fitting bolt, and in the mounting process, hydraulic pressure is used for enabling a tapered bushing to be expanded, so that interference fit of the hydraulic expansion bolt and the surface of a connecting hole is achieved, torque transmission is ensured, in the dismounting process, the hydraulic pressure is used for enabling the conical bushing to be restored, the clearance fit between hole shafts is achieved, and the damage to the inner wall of the connecting hole is avoided. According to the device, nondestructively disassembling is achieved, the scrap returning rate is reduced, and the advantages of being easy and convenient to dismount and mount, high in operation efficiency, high in tool generality and long in service life are achieved. The device is suitable for manufacturing and mounting of all large type ship low-speed diesel engines with segmented crankshaft structure.